This invention relates to pigments and, more particularly, this invention relates to water-insoluble pigments which are prepared from water-soluble dyes, and methods for making such pigments.
Pigments and dyes are known in the art as coloring agents and are useful in various applications such as for coloring cosmetics, soaps, food, paints, plastics and polymers. Dyes are generally liquids or soluble solids which are used in solution. Pigments, conversely, are generally solids and are usually insoluble in the medium in which the pigment is being used. Pigments are typically preferred over dyes in applications where color migration or bleeding is undesirable. For example, if a dye is used in a toothpaste formulation, the dye may be absorbed by the tongue, teeth and gums of the user. Similarly, in a two tone bar soap, a dye in one color section of the soap may migrate to a different color portion of the soap. Further, the soap dyes may stain the skin of the user as well as sink fixtures. The use of an insoluble pigment, in these applications, prevents these undesirable occurrences.
Although there are a number of known water-insoluble pigments, some are considered to be unsuitable because they have been found to be questionable by government regulators on the ground of potential toxicity or carcinogenicity to the user. In addition, some insoluble pigments which are government-approved may suffer from a lack of color flexibility. These problems vary according to the use to which the pigments are put. In the case of soaps and cosmetics, for example, skin irritation may result from the incorporation of some known pigments.
Another issue involved is that of coloration of plastics that will be used in contact with food. Pending United States Food and Drug Administration (FDA) regulation will require certification of any colorant used in contact with food, but current plastics colorants are for the most part unlikely to qualify for such certification, and dyes and pigments currently approved for food contact do not generally exhibit the chemical and thermal stability necessary for plastics processing.
A final problem encountered in the pigment and dye area is related to commercial desirability. In producing pigments from dyes, a loss of color brightness and intensity, along with hue changes, is encountered. While use of increased amounts of pigments will help to counteract these effects, the expensive cost of the dye precursor represents significant increases in costs associated with pigment production and, ultimately, of the soap, cosmetic, food, plastic or other polymer in which the colorant will be utilized.
Thus, it would be highly desirable to provide a nontoxic, noncarcinogenic, water-insoluble pigment which is suitable for use in body contact and plastics processing applications and which has a relatively greater color flexibility. It would also be desirable to provide a method of preparing such a nontoxic, noncarcinogenic, chemically and thermally stable water-insoluble pigment.